Now updated with answers :-)
Where was I last week?
Congratulations to those of you who guessed! Yes, I was in Athens. What you see in this photo is not the Parthenon, but a view of the Thisseion area from the Acropolis. From left to right you can see the Temple of Hephaestus, the Ancient Agora (or rather what's left of it - the pillars standing in a row) and the church of the Holy Apostles (Agii Apostoli). The expanse of cement in the background is the modern city of Athens.
And, most importantly, what did I do while I was there?
Oh yes, none of you guessed but I did go swimming. Twice. The weather was extremely mild for early November (24-25C during the day) and the water still warm from summer, and it was beautiful. When I booked my flights, the best I'd hoped for was T-shirt weather. Instead, what we got resembled late spring and we just couldn't resist taking advantage to lie in the sun and take a dip in the sea. Not just one dip either, we were in the water for over an hour on the first day and a good 40 minutes on the second day. The glorious weather didn't last, but it was still quite warm (well, compared to London) for the rest of our stay, never falling below 16C and not a drop of rain either.
The photo was taken in Glyfada, 30 mins drive from the centre of Athens, but in the end we swam a bit further down the coast (Vouliagmeni and Varkiza, for those in the know).